I've built several kind of amps, from fully symmetrical design to single differential design. All have turn on and turn off thump.
Searching here, the most encouraged cure is to use output relays.
But some good commercial amps do not have these thumps, although they do not use delay relay at all.
1. What is the mechanism of this Thump generation?
2. Is there any trick if we design an amp so it will not have thump?

Those thumps are usually caused by capacitors placed in the signal plath or in biasing networks. At power-on those capacitors are discharged and they take some time to charge to its steady state conditions. Most circuits tend to show undesired behavior until these capacitors are charged

In the same vein as what Eva was saying, I recently had cause to pull out an old Dyna PAS tube preamp to build a small system for my wife's use. I had never used the preamp; it had been donated years ago by a friend. I thought that it would be wise to check it out on the bench before pressing it into service. I replaced the aged electrolytics and turned it on.
No thump.
The secret? Tube rectification. The rails come up very slowly, indeed.
(The preamp worked fine, by the way.)
It's not difficult to design regulators that incorporate a slow start. That way you get a fringe benefit by bringing up the rails at a rate that allows the circuit to settle.

I am thinking if we can left the circuit in "stand by mode", something alike remote controled television set...but much more simple.

I imagine one resistor in series with transformer primary....resulting in half voltage, or less than half voltage... circuit always on. To switch circuit to power work, you make a short circuit over the resistor leads, making a switched jumper on it. Hey!, i never did that!....do not have sure if will work!

Other simple, and cheap idea is to increase the condenser value making voltage divider to supply driver transistor and bias circuit....making the big cap charge slowly.... this way, the thump will be there, but will be interesting and slow...you will see the speaker move slowly up and down till stop in a stable form.

The third one may be the best.... use a relay and a big condenser in parallel with the coil.... feed this relay througth a resistor.... speaker will be conected to output post when the rellay became on.... the RC will produce the time needed to amplifiers stabilize... when really ready...no more fluctuations....this one seems, to me, to be very good....because no complicated circuits, and you can also use other relay contacts to switch on a fan and to make some led show you when ready or not ready.... many people love those lamps in the amplifier panel, but not my case, ligths flashing to me, only in Christmas time.

If you regulate the rails for the driver and amp circuit(excluding the output collectors) then you can design a transistor swicth to switch both the + & - rails on and off using say a couple of comparators or op-amps. You can then use a logic +5V to turn on and off the whole amp circuit. The collectors of the outputs will not have any current flowing in them when the drivers are off. Then you can have another transistor switch to switch on a "bleeding resistor" to discharge the filter caps slowly so there will never be any loose current to create a 'thump'. You may need to have a very small AC power transformer to run the digital circuitry.

BTW: Using this tecnique you can create a delay when turning the amp on and allow the filter caps to fully charge when the rails are switched on. If flip-flops are used in digital circuit, cheap reliable momentary switches can be used. Also you may include a 'fault' circuit in your design to switch off the amp if say a fuse blows or current sensor is triped or something like that.

I have melted my fair share of outputs and some are quite expensive. I have developed some circuits that have saved me money many times and needs not a relay.

Hi lumanauw,
The basic trick is to have the voltage amp stabilise before the output stage is activated.
As long as the voltage amp runs after the power to the output stage dies there will be no turn off thump either.
My personal believe is that an output relay is a great way to protect your speakers. Therefore, not a bad thing. Relay contacts in good condition affect the sound less than a fuse. (one more contact and a thermally variable resistor)
-Chris

Normally they are produce complex ideas, because their mind full of options...never go to an easy solution.
Cuningham is one famous man, have wonderfull knowledge, is respected as one of the bests we have in forum.
But unfortunattely they do not use the straigth line as the smaller distance between two points anymore... they have so big knowledge that they already realise the "straigth line", do not exist in our universe.... everything is curved, have turns, because a straigth line over the earth is never flat, because earth is rounded...all spatial travels made considered that aspect, from gravity atractions of big masses in space..... they considered the non possibility of a straight travel line...... also a flat line made with a pen in a paper is not a line....is a lot of points over the paper "peaks"..... when you go closer to a line, you will se that is not straigth....nothing is perfect aligned in our universe.

Those deep knowledges, makes people ideas always more complex... this way, Mr. Cunningham, beeing an expert in electronics, will never use a damn relay with stupid contacts... this is too much simple.... will use many parts to make this job...will make it better, maybe a little bit more expensive, but will make it better.

Unfortunattely, in audio amplifiers, each stage create its problem to the final result.... some noise, some disturbance.... and the more crazy is that.... the biggest the knowledge, the more complicated the circuit, with too much stages creating their problems, and the next stage trying to compensate something damaged in early stages... this way...enormous circuits, and the sound....normally strange.

Those crazy things in the life... the best the enginner.... normally, with many exceptions, i believe Cuningham can be one of them, but normally their products sound strange.

The techonologie, is so much advanced that is creating some problems..... we can see 2 milions transistors to control some switching functions and lamps flashing on panels.... or displays...

You go on deep, and you will see that the signal patch, normally not many transistors doing the real work..... and a lot of complications come together... with one with their probability of failure.... resulting some miracle working... we never know...how long!.

Strange those things.... by this reason i am running from Medicine Doctors....in special the best doctors.....they will complicate all my life... some of them can kill me..... i am running fast as i can from medicine doctors.

Why not a capacitor or a relay... to delay condenser charge, making infrasonic speaker movement?.....because is too much simple.

Originally posted by destroyer X
Those deep knowledges, makes people ideas always more complex... this way, Mr. Cunningham, beeing an expert in electronics, will never use a damn relay with stupid contacts... this is too much simple.... will use many parts to make this job...will make it better, maybe a little bit more expensive, but will make it better.

Unfortunattely, in audio amplifiers, each stage create its problem to the final result.... some noise, some disturbance.... and the more crazy is that.... the biggest the knowledge, the more complicated the circuit, with too much stages creating their problems, and the next stage trying to compensate something damaged in early stages... this way...enormous circuits, and the sound....normally strange.

You go on deep, and you will see that the signal patch, normally not many transistors doing the real work..... and a lot of complications come together... with one with their probability of failure.... resulting some miracle working... we never know...how long!.

Why not a capacitor or a relay... to delay condenser charge, making infrasonic speaker movement?.....because is too much simple.

regards,

Carlos

Hi Carlos

I have found that over time relay contacts tend to corrode causing resistance at the contact point. This = voltage drop X currnet, and can cause bad contacts and horrible sound. This can be fixed by plating the contacts with gold. Unfortunatly, this is expensive. I have done many experiments with NOT using a relay for two reasons.
1) I already designed my amp circuit with a regulated power supply for all but the output drivers(collectors) so making it switchable was not that much more complicated. Just turn off the regulators. None of these added componants are in the signal path. Tis not that complicated, just simple electronic circuits put together in redundancy.

2) 5 surface mount transistors, one quad comparator chip, and a few small surface mount resistors are cheaper than buying a relay large enough to break a peak of 7A and not fuse contacts together. We are talking just a couple of dollars. Maybe the circuit construction is more complex, but I am only making one unit for my self and complexity really doesn't bother me too much.

I started with a simple amp circuit and just have continuously figured ways to improve. However, on this path, I may never actually finish it.